Orexin (or hypocretin) signaling is mediated by two receptors and two peptide agonists. The two orexin peptides (orexin A and orexin B) herein after referred to as orexins, bind to two high affinity receptors, termed Orexin-1 and Orexin-2 receptors. The Orexin-1 receptor is selective in favour of orexin A, while the Orexin-2 receptor binds both orexins with similar affinities. The orexins, are cleavage products of the same gene, prepro-orexin. In the central nervous system neurons expressing prepro-orexin, the precursor from which orexin is produced, are found in the perifornical nucleus, the dorsal hypothalamus and the lateral hypothalamus (C. Peyron et al., J. Neurosci., 1998, 18(23), 9996-10015). Orexinergic cells in these nuclei project to many areas of the brain, extending rostrally to the olfactory bulbs and caudally to the spinal cord (van den Pol, A. N. et al., J. Neuroscience., 1999, 19(8), 3171-3182).
The broad CNS distribution of orexin projections and neurons expressing orexin receptors is suggestive of orexin involvement in a number of physiological functions including; feeding, drinking, arousal, stress, reward, metabolism and reproduction (T. Sakurai, Nature Reviews Neuroscience, 2007, 8(3), 171-181). The targeted necrosis of cells expressing prepro-orexin suggests the most physiologically important roles of the orexins are likely to be effects on arousal, feeding and metabolism (J. Hara et al., Neuron, 2001, 30, 345-354). A prominent orexin neuronal projection via the vagus nerve probably mediates central orexin effects on cardiac parameters (W. K. Samson et al., Brain Res., 1999, 831, 248-253; T. Shirasaka et al., Am. J. Physiol., 1999, 277, R1780-R1785; C.-T. Chen et al., Am. J. Physiol., 2000, 278, R692-R697), gastric acid secretion and gastric motility (A. L. Kirchgessner and M.-T. Liu, Neuron, 1999, 24, 941-951; N. Takahashi et al., Biochem. Biophys. Res. Commun., 1999, 254, 623-627).
Several lines of evidence indicate that the orexin system is an important modulator of arousal. Rodents administered orexins intracerebroventricularly spend more time awake (Piper et al., J. Neurosci. 2000, 12, 726-730). Orexin mediated effects on arousal have been linked to orexin neuronal projections to histaminergic neurons in the tuberomammillary nucleus (TMN) (Yamanaka et al., Biochem. Biophys. Res. Comm. 2002, 290, 1237-1245). TMN neurons express the orexin-2 receptor primarily, and the orexin-1 receptor to a lesser extent. Rodents whose prepro-orexin gene has been knocked out, or whose orexigenic neurons have been lesioned, display altered sleep/wake cycles similar to narcolepsy (Chemelli et al., Cell 1999, 98, 437-451; Hara et al., 2001, supra). Dog models of narcolepsy have been shown to have mutant or nonfunctional orexin-2 receptors (Lin et al., Cell 1999, 98, 365-376). Human narcolepsy appears to be linked to deficient orexin signalling, likely related to immune ablation of orexinergic neurons in the lateral hypothalamus (Mignot et al., Am. J. Hum. Genet. 2001, 68: 686-699; Minot & Thorsby, New England J. Med. 2001, 344, 692), or, in rare cases, to mutations in the orexin-2 gene (Peyron et al., Nature Med. 2000, 6, 991-997). The disclosure that rats, dogs and humans treated with the dual orexin-1/2 receptor antagonist, ACT-078573 (Brisbare-Roch et al., Nature Medicine, 2007, 13, 150-155) exhibited decreased alertness together with characteristic clinical and EEG (electroencephalographic) signs of sleep provides evidence to support a role for the orexin system in the regulation of arousal, sleep and wake states. EEG data indicates that orexin-2 may be more important than orexin-1 in the modulation of sleep/wake (P. Malherbe et al., Molecular Pharmacology (2009) 76(3):618-31; C. Dugovic et al., J. Pharmacol. Exp. Ther., 2009, 330(1), 142-151). Disorders of the sleep-wake cycle are therefore likely targets for orexin-2 receptor antagonist therapy. Examples of such disorders include sleep-wake transition disorders, insomnia, restless legs syndrome, jet-lag, disturbed sleep, and sleep disorders secondary to neurological disorders (e.g., manias, depressions, manic depression, schizophrenia, and pain syndromes (e.g., fibromyalgia, neuropathic pain). The orexin system also interacts with brain dopamine systems. Intracerebroventricular injections of orexins in mice increase locomotor activity, grooming and stereotypy; these behavioural effects are reversed by administration of D2 dopamine receptor antagonists (Nakamura et al., Brain Research, 873(1), 181-7). Therefore, orexin-2 modulators may be useful to treat various neurological disorders; e.g., agonists or up-regulators to treat catatonia, antagonists or down-regulators to treat Parkinson's disease, Tourette's syndrome, anxiety, delirium and dementias. Recent evidence indicates a role for orexin in the pathogenesis of Alzheimer disease (Kang et al, Science Express, 2009, 1-10). Brain interstitial fluid levels of amyloid-beta were demonstrated to fluctuate diurnally in both humans and rodents with sleep deprivation in rodents leading to significant increases in brain interstitial fluid levels of amyloid-beta. Infusion of a dual orexin antagonist in rodents suppressed interstitial levels of amyloid-beta and abolished the natural diurnal variation of amyloid-beta. The reduction of interstitial fluid amyloid-beta levels is correlated with reduced amyloid plaque formation, a hallmark of Alzheimer's disease, and consequently the regulation of sleep time could potentially inhibit amyloid-beta aggregation and slow the progression of Alzheimer's disease. Orexin neurons project to many regions of the brain associated with reward function (T. Sakurai, supra) and research, focusing on animal models of drug intake, reward, and reinstatement, has expanded the link between the orexin system and addiction. A comprehensive set of data suggest that drugs of abuse activate the orexin system, which in turn enhances drug reward or drug seeking (G. Aston-Jones et al., Neuropharmacology, 2009, 56 (Suppl 1) 1 12-121. Thus interactions between nicotine (J. K. Kane et al., Endocrinology, 2000, 141 (10), 3623-3629; J. K. Kane et al., Neurosci. Lett, 2001, 298(1), 1-4), morphine (D. Georgescu, et al., J. Neurosci., 2003, 23(8), 3106-3111) and amphetamine (C. J. Winrow et al., Neuropharmacology, 2010, 58(1), 185-94) and the orexin system have been demonstrated. Additional studies from a number of laboratories have demonstrated an important relationship between the Orexin system and ethanol consumption. As examples, ethanol consumption in an alcohol-preferring strain of rat was shown to up regulate Orexin mRNA in the lateral hypothalamus and that an Orexin-1 receptor antagonist reduced operant responding for ethanol (Lawrence, et. al., Br. J. Pharmacol., 2006, 148, 752-759). Treatment with an orexin-1 antagonist has also been shown to decrease operant responding for ethanol (Richards, et. al., Psychopharmacology, 2008, 199 (1), 109-117). Other studies have demonstrated increased Fos activation of orexin neurons following contextual reinstatement to ethanol seeking (Dayas, et. al., Biol. Psychiatry, 2008, 63 (2), 152-157 and Hamlin, et. al., Neuroscience, 2007, 146, 525-536). Studies have also shown increased ethanol consumption following Orexin infusion into the paraventricular nucleus of the hypothalamus or in the lateral hypothalamus (Schneider, et. al., Alcohol. Clin. Exp. Res., 2007, 37(11), 1858-1865). These studies provide evidence that modulation of the Orexin system effects alcohol preference and therefore Orexin receptor antagonists are likely to be useful for the treatment of alcoholism.
Orexins and their receptors have been found in both the myenteric and submucosal plexus of the enteric nervous system, where orexins have been shown to increase motility in vitro (Kirchgessner & Liu, Neuron 1999, 24, 941-951) and to stimulate gastric acid secretion in vitro (Takahashi et al., Biochem. Biophys. Res. Comm. 1999, 254, 623-627). Orexin mediated effects on the gut may be driven by a projection via the vagus nerve (van den Pol, 1999, supra), as vagotomy or atropine prevent the effect of an intracerebroventricular injection of orexin on gastric acid secretion (Takahashi et al., 1999, supra). Orexin receptor antagonists or other down-regulators of orexin receptor-mediated systems are therefore potential treatments for ulcers, irritable bowel syndrome, diarrhea and gastroesophageal reflux. Body weight may also be affected by orexin-mediated regulation of appetite and metabolism (T. Sakurai et al., Cell, 1998, 92(4), 573-585; T. Sakurai, Reg. Pept, 1999, 85(1), 25-30). Some effects of orexin on metabolism and appetite may be mediated in the gut, where, as mentioned, orexins alter gastric motility and gastric acid secretion. Orexin receptor antagonists therefore are likely to be useful in treatment of overweight or obesity and conditions related to overweight or obesity, such as insulin resistance, type II diabetes, hyperlipidemia, gallstones, angina, hypertension, breathlessness, tachycardia, infertility, sleep apnea, back and joint pain, varicose veins and osteoarthritis. Conversely, orexin receptor agonists are likely to be useful in treatment of underweight and related conditions such as hypotension, bradycardia, amenorrhea and related infertility, and eating disorders such as anorexia and bulimia. Intracerebroventricularly administered orexins have been shown to increase mean arterial pressure and heart rate in freely moving (awake) animals (Samson et al., Brain Res. 1999, 831, 248-253; Shirasaka et al., Am. J. Physiol. 1999, 277, R1780-R1785) and in urethane-anesthetized animals (Chen et al., Am. J. Physiol. 2000, 278, R692-R697), with similar results.
Orexin receptor agonists may therefore be candidates for treatment of hypotension, bradycardia and heart failure related thereto, while orexin receptor antagonists may be useful for treatment of hypertension, tachycardia and other arrhythmias, angina pectoris and acute heart failure.
From the foregoing discussion, it can be seen that the identification of orexin receptor antagonists, in one embodiment modulators of the orexin-2 receptor, will be of great advantage in the development of therapeutic agents for the treatment of a wide variety of disorders that are mediated through these receptor systems.
Certain orexin antagonists are disclosed in PCT patent applications: WO2010/0480116, WO2010/051238, WO2006/127550, WO2010/060470, WO2010/060471, WO2003/051368, WO2011/076747 and WO2009/016564. There remains a need, however, for potent orexin dual receptor antagonists with desirable pharmaceutical properties
The object of the present invention is to provide 2-azabicyclo[4.1.0]heptane compounds with dual antagonist activity at the Orexin 1 and Orexin 2 receptors.